The present invention relates to a thick film circuit board and more particularly, to a thick film circuit board having a thick film resistor thereon. The present invention also relates to a method of manufacturing the thick film circuit.
FIGS. 9 to 12 are perspective views showing a known process of manufacturing a known thick film circuit board. As shown in FIG. 9, wiring electrodes 2 are formed on a substrate 1 made of, for example, ceramic by a printing and baking method. An electrically conductive material including silver and palladium is used for the wiring electrodes 2. As shown in FIG. 10, a thick film resistor 3 is printed and baked in such a manner that the resistor 3 is connected to the and bridges the two wiring electrodes 2. Ruthenium oxide is used for the thick film resistor 3. Since the resistance of the thick film resistor 3 depends upon the material, width, length and thickness of the thick film resistor 3, the resistance can be designed on the basis of them. As shown in FIG. 11, an overcoat 4 made of glass having a low melting point coats the entire surface of the substrate 1 excluding portions to be exposed outside on which elements are mounted. As the wiring electrodes 2 are made of an electrically conductive material, if water intrudes into the wiring electrodes 2, silver in the wiring electrodes 2 is ionized, so that an undesirable electrical path is occasionally established between the wiring electrodes 2. This is referred to as "migration" between wiring electrodes. The overcoat 4 is formed mainly for preventing the migration between the wiring electrodes 2, i.e., preventing the thick film circuit board from being electrically damaged. However, because the overcoat 4 is fragile as a characteristic of glass and weak against an external mechanical force, the thick film resistor 3 is often damaged when it is hit against something. In this meaning, the overcoat 4 is insufficient to protect the thick film resistor 3 from mechanical damage.
As described above, the thick film resistor 3 is formed by the printing and baking method. Therefore, the resistance of the thick film resistor 3 after the printing and baking changes greatly depending upon the deviation of thickness or width and the unevenness of material during the printing. For this reason, the thick film resistor 3 has been printed in such a manner that the resistor 3 has a resistance smaller than a target resistance and then a part of the thick film resistor 3 is heated and vaporized by a laser beam and removed together with the overcoat 4 such that the resistance increases gradually, so that the resistance of the resistor 3 can be adjusted to match to the target resistance. The adjusting process of the resistance of the thick film resistor 3 is referred to as the "trimming process". FIG. 12 shows a trimming trace 3a. The trimming process is classified into two types. In one type of trimming process, the resistance of the thick film resistor 3 is adjusted before an assembly process considering only the resistance of resistor 3. In another type of trimming process, the resistance of the thick film resistor 3 is adjusted while the output characteristic of all or part of a circuit on the substrate 1 is measured and monitored so that the deviation of characteristic of mounted elements can be eliminated, after a semiconductor element is mounted on the substrate 1 in the assembly process. The other type of trimming process is referred specifically to as "function trimming". As another trimming method, there is a sand trimming method in which silicon powder is sprayed with compressed air to scour the resistor 3.
The known thick film circuit board is structured and manufactured as described above and the resistance of the thick film resistor 3 is adjusted to proper target value. However, when a damage 3b is given to the surface of the thick film resistor 3 later, as shown in FIG. 13, the resistance changes, resulting in a faulty part. Specifically, in a case that a high precision is required, little damage also has a great effect.
In a manufacturing process after the trimming process, a plurality of substrates 1 are individually put into slits in a stand such that they are arranged and transferred in the standing state. In this case, when one of the plurality of substrates 1 falls on an adjacent substrate 1, if the edge of the fallen substrate 1 strikes the surface of the thick film resistor 3 of the adjacent substrate 1, the surface of the adjacent substrate 1 is damaged because a material having a great hardness, such as ceramic, is mainly used for the substrate 1, as described above.
Also, in the manufacturing process, a plurality of substrates 1 are arranged on a flat plate in a horizontal state and in a matrix. In this case, if the flat plate collides with something and receives a force, some of the plurality of substrates 1 located on the flat plate in the horizontal state often move onto adjacent substrates 1. At this time, if the edge of the substrate 1 strikes the surface of the thick film resistor 3 of another adjacent substrate 1, the adjacent substrate 1 is also damaged.
Recently, a thick film circuit board has been developed to have the structure in which circuits are arranged on the front and back surfaces as shown in FIGS. 14 and 15, i.e., the structure in which parts such as a chip element 5a and a semiconductor element 5b are arranged on the front surface of the substrate 1 while the thick film resistor 3 is arranged on the back surface, and a through hole 1a is provided in the substrate 1 to electrically connect between wiring electrodes 2 on both the front and back surfaces of the substrate 1. In such a structure of the thick film circuit board, the substrate 1 is put on the surface of a plate such that the back surface of the substrate 1 is in contact with the plate surface. When there is something undesirable on the plate surface, the thick film resistor 3 is scoured damaged. In a case where a plurality of substrates 1 having the above structure are put into slits of a stand in a vertical state and transferred as described above, if one substrate 1 falls on another adjacent substrate 1 so that an element mounted thereon strikes the surface of the thick film resistor 3 mounted on the back surface of the other adjacent substrate 1, the resistor 3 is often damaged. As described above, in the thick film circuit substrate 1 of the structure in which the thick film resistor 3 is arranged on the back surface of the substrate 1, because there is a high probability that something like a contaminant will strike the thick film resistor 3 mounted on the back surface of the substrate 1, the thick film resistor 3 is possibly damaged. Thus, a new problem is caused in that a fault in the thick film resistor 3 is caused by mechanical damage in many cases.
In order to prevent the thick film resistor 3 from being damaged, it could be considered to overcoat the whole thick film resistor 3. However, when the overcoat is thin, the protection against an external mechanical force is not sufficient. Thus, a thick overcoat is required. In this case, however, laser beam or silicon powder trimming is resisted by the thick overcoat and satisfactory trimming cannot be performed. In addition, products employing a heat resisting ceramic substrate 1 are used in many cases under great temperature change, such as in the engine compartment of an automobile. When the overcoat is thick, the thermal stress of the overcoat is greater. A large temperature change in a connecting portion of the thick film resistor 3 may peel the resistor 3 from the substrate 1.